The present invention relates to III-V compound semiconductor devices represented by semiconductor laser devices used as optical information system light sources for CD, MD and DVD players or computer information storage devices, and to a manufacturing method thereof. The present invention relates, in particular, to a structure for achieving a low threshold current operation, to a semiconductor device excellent in device characteristics, yield and reliability with improved controllability of impurities included in semiconductor layers, and to a manufacturing method thereof.
In recent years, there has been a growing demand for semiconductor laser devices that are compound semiconductor devices to be used for the pickups of CD and MD. Semiconductor laser devices that have little characteristic variations and excellent reliability have been demanded. Also, it is anticipated that the demand for semiconductor laser devices will be still more increasing in future for the production of the computer information storage devices such as CD-ROM, CD-R, CD-RW, and digital video discs (DVD).
When producing a III-V compound semiconductor device represented by such a semiconductor laser device, a stacked structure of a plurality of semiconductor layers is formed on a semiconductor substrate. By adding a specified impurity to each semiconductor layer, the electric conduction type or the electric conductivity of each layer is controlled to consequently obtain a device of specified semiconductor characteristics. To achieve uniform device characteristics of the semiconductor lasers and improvement in yield of products, it is very important to control the electric conduction type or the electric conductivity of each layer of the semiconductor device to be in conformity with designed values.
As a method of forming III-V compound semiconductor thin films in a stacked manner, the MOCVD (metal-organic chemical vapor deposition) method and the MBE (molecular beam epitaxy) method can be mentioned. When growing a film by using any of these methods, a group IV element such as silicon (Si) and a group VI element such as selenium (Se) are used as impurities for obtaining an n-type electric conduction type layer. The group IV element becomes a donor impurity by replacing a group III element of aluminum (Al), gallium (Ga), or indium (In). The group VI element becomes a donor impurity by replacing a group V element of arsenic (As) or phosphorus (P). On the other hand, as an impurity for obtaining a p-type electric conduction layer, a group II element such as zinc (Zn), beryllium (Be), or magnesium (Mg) is employed. The group II element becomes an acceptor impurity by replacing a group III element of Al or Ga.
Among semiconductor laser device structures, what we call a self-alignment structure and what we call a ridged structure are well known. FIGS. 4A, 4B and 4C show an example of a semiconductor laser device of the self-alignment structure. The fabricating process of this semiconductor laser device will be described below.
In the first process step shown in FIG. 4A, first, an n-type GaAs buffer layer 12 (layer thickness: 0.5 xcexcm), an n-type AlxGa1xe2x88x92xAs first cladding layer 13 (x=0.5, layer thickness: 1.0 xcexcm), a non-doped AlxGa1xe2x88x92xAs active layer 14 (x=0.14, layer thickness: 0.085 xcexcm), a p-type AlxGa1xe2x88x92xAs second cladding layer 15 (x=0.5, layer thickness: 0.35 xcexcm) and an n-type GaAs current block layer 16 (layer thickness: 0.6 xcexcm) are successively grown on an n-type GaAs substrate 10 by the MOCVD method. In this stage, Se is employed as the n-type impurity, while Zn is employed as the p-type impurity. Next, in the second process step shown in FIG. 4B, an etching mask 40 is formed by a method such as photolithography. Thereafter, the n-type GaAs current block layer 16 is removed in a stripe-like and groove-like shape with a width of 3.5 to 4.0 xcexcm, forming a removed portion 20.
Subsequently, in the third process step shown in FIG. 4C, a p-type AlxGa1xe2x88x92xAs third cladding layer 17 (x=0.5, layer thickness: 1.0 xcexcm) and a p-type GaAs cap layer 18 (layer thickness: 3 to 50 xcexcm) are grown on the n-type GaAs current block layer 16 including the removed portion 20 by the MOCVD method or the LPE method. In this case, the layer thickness of the p-type GaAs cap layer 18 should be determined as the occasion demands depending on where the final light emitting point of the semiconductor laser device is to be positioned relative to the chip thickness. Zn or Mg is employed then as the p-type impurity. By the aforementioned fabricating method, the semiconductor laser device of the self-alignment structure is obtained.
The molar ratio of the group V element to the group III element (V/III ratio) when forming a laminate by the MOCVD method in the first process step has conventionally been set to 20 to 150 at a growth temperature of 600xc2x0 C. to 800xc2x0 C. If the ratio is set to a value of 20 or lower, then there occurs a phenomenon of roughened growth surface. On the other hand, it has been reported that if the growth temperature is set to 450xc2x0 C. to 600xc2x0 C., then no roughness occurs on the crystal surface even when the V/III molar ratio is reduced to 0.3 to 2.5, and that the intake of carbon C to the grown thin film is increased so that a p-type hole density of 1xc3x971018 cmxe2x88x923 to 1xc3x971020 cmxe2x88x923 by the carbon C of GaAs and AlGaAs is obtained (JP-B2-2885435).
In a practically used semiconductor laser device of the structure shown in FIG. 4C, in the first process step for forming at least the n-type first cladding layer 13, the active layer 14, the second cladding layer 15 and the n-type current block layer 16 on the n-type GaAs substrate, the n-type first cladding layer 13 and the n-type current block layer 16 are doped with an impurity of Se, and the p-type second cladding layer 15 is doped with an impurity of Zn However, in the structure after the completion of the first process step, the impurity elements move or migrate between the layers by diffusion or the interaction of the impurity atoms during the fabricating process, which results in an impurity profile different from a designed impurity profile. FIG. 3A shows the designed impurity concentration profile, in which, of course, the n-type first cladding layer 13 and the n-type current block layer 16 are designed to be doped with the n-type impurity of Se, and the p-type second cladding layer 15 is designed to be doped with the p-type impurity of Zn, each with a steep doping slope. FIG. 3B shows an actual impurity concentration profile. As obvious from this figure, the impurity of Zn in the p-type second cladding layer 15 diffuses into the layers other than the p-type second cladding layer 15 during the growth of the n-type current block layer 16 in the first process step, as a consequence of which the doping control of the p-type second cladding layer 15 becomes unstable.
Further, in the third process step after the formation of the stripe removed portion 20 in the n-type current block layer 16 in the second process step, due to a thermal history during the process for growing the p-type third cladding layer 17 end the p-type GaAs cap layer 18 at the removed portion of the current block layer 16 and the non-removed portion of the current block layer, the impurity of Zn in the p-type second cladding layer 15 increasingly diffuses into the other layers and, in certain circumstances, the impurity of Se of the n-type first cladding layer 13 and the n-type current block layer 16 diffuses into the p-type second cladding layer 15. The diffusion of n-type impurity surpasses the concentration of the p-type impurity of Zn of the p-AlGaAs cladding layer 15, consequently causing the inversion of the p-type second cladding layer 15 into the n-type. This inversion into the n-type, which occurs either on the entire surface of the p-type second cladding layer 15 or in the portion that faces the non-removed portion of the n-type current block layer 16, disables the local current injection for obtaining the laser oscillation of the semiconductor laser device, causing a defective product.
Also, in the ridged-structure semiconductor laser device as well, Zn has been employed as the impurity added to the p-type cladding layer to be formed on an active layer constructed of a quantum well layer. Therefore, similar to the semiconductor laser device of the self-alignment structure, Zn disadvantageously diffuses into the active layer during the fabricating process. This causes the disorder of the entire quantum well active layer, eventually changing the oscillation wavelength. The diffusion otherwise causes the degradation in crystallinity of the quantum well active layer, disadvantageously increasing the threshold current and the operating current. This has resulted in degradation in laser characteristics and an increase in characteristic variations.
Accordingly, an object of the present invention is to provide a fabricating method capable of doping a p-type cladding layer, in particular, a p-type cladding layer formed in the vicinity of the active layer in conformity with the designed impurity concentration and with good controllability, and also provide a semiconductor laser device structure formed by the method.
There is provided, according to an aspect of the invention, a semiconductor laser device in which at least an n-type first cladding layer, an active layer, and a p-type second cladding layer are stacked on or above an n-type semiconductor substrate, wherein the second cladding layer contains carbon as a p-type impurity at a concentration of 2xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923.
Carbon is an impurity that diffuses very little. Therefore, the electric conduction type of the p-type second cladding layer is prevented from being inverted to the n-type. Thus, the semiconductor laser device of the present invention is allowed to have an impurity concentration profile almost as designed.
In one embodiment, an n-type current block layer having a stripe-shaped groove is provided on or above the second cladding layer, at least a p-type third cladding layer is formed on the current block layer including the stripe-shaped groove, and the carbon concentration of the second cladding layer is 3xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923. This semiconductor laser device is of what we call a self-alignment structure.
To fabricate this semiconductor laser device, it is possible to use a method comprising:
a first process step of successively forming at least a first cladding layer, an active layer, a second cladding layer, and a current block layer on or above a semiconductor substrate, wherein a growth condition for the second cladding layer is such that the second cladding layer has carbon serving as a p-type impurity at a concentration of 3xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923;
a second process step of forming a stripe-shaped groove in the current block layer; and
a third process step of forming at least a third cladding layer on the current block layer including the stripe-shaped groove.
This fabricating method makes it hard for the impurity to diffuse from the second cladding layer into other layers and vice versa during the growth of the n-type current block layer in the first process step and during the formation of at least the third cladding layer in the third process step. Therefore, the electric conduction type of the p-type second cladding layer can be prevented from being inverted to the n-type.
The third process step may comprise diffusing an impurity from the third cladding layer into the second cladding layer via the groove of the current block layer such that the impurity concentration of the second cladding layer is higher in a region that faces the groove of the current block layer than in the remaining regions.
In the thus-obtained semiconductor laser device, the second cladding layer has a lower resistance in its higher impurity concentration portion which faces a removed portion, i.e., the stripe-shaped groove of the current block layer than in its lower impurity concentration portions that are the remaining regions of the second cladding layer. Therefore, a current that reaches the higher impurity concentration portion of the second cladding layer is prevented from diffusing into the lower impurity concentration portions. Therefore, according to the present invention, a wattless current, which would otherwise occur due to the current diffusion inside the second cladding layer, is prevented from occurring. This, a low threshold current and a low current drive operation are achievable.
In one embodiment, in the first process step, a metal-organic chemical vapor deposition method is used, and the growth condition for the p-type second cladding layer is that a molar ratio of a group V source to a group III source (V/III ratio) is 20 to 50. With this arrangement, the possible occurrence of surface roughness of the grown crystal can be avoided, and the carbon (C) concentration becomes 3xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923. Use of this fabricating method enables, with controllability and reproducibility superior to those in the conventional case, the p-type second cladding layer to have an impurity concentration that causes no inversion of the electric conduction type.
In the third process step, liquid phase epitaxy (LPE) can be used instead. In this case, adjusting the temperature and time of the LPE growth achieves diffusion of an optimum quantity of impurity into the second cladding layer that faces the removed portion of the current block layer. If the light emitting position of the semiconductor laser device is required to be a central position with respect to the final chip thickness of, for example, 100 xcexcm, then the cap layer on the third cladding layer must be grown to a thickness of 50 xcexcm in the third process step. The LPE method achieves this in a short time and at low cost.
By using magnesium (Mg) as a p-type impurity for the third cladding layer in the third process step using the LPE method, diffusion of an optimum quality of impurity into the second cladding layer that faces the removed portion of the current block layer can be easily performed.
In the comparison between the semiconductor laser device having undergone the LPE in the third process to diffuse Mg and the semiconductor laser device having undergone the MOCVD in the third process to diffuse no impurity, the threshold current of the latter was 42 mA, which was 7 mA greater than that of the former. In addition, the former had a drive voltage of 1.88 V at the optical output power of 5 mW, while the latter had a drive voltage of 1.95 V, meaning that the latter had a larger device resistance than the former. This fact proved the superiority of the structure in which the impurity concentration of the second cladding layer in the region that faces the groove is made higher than the impurity concentration of the second cladding layer in the remaining regions.
According to the present invention, carbon, which diffuses very little, is used for the second cladding layer. Therefore, even if the n-type impurity of Se is used in growing n-type semiconductor layers on the substrate, the diffusion of the impurity of Se from the first cladding layer and the current block layer into the second cladding layer can be effectively prevented.
In another embodiment, the active layer comprises a quantum well layer or layers, and the semiconductor laser device further comprises a ridge-shaped p-type third cladding layer and an n-type current block layer which are provided on or above-the second cladding layer in a manner that the third cladding layer extends in a stripe-like shape and that the n-type current block layer holds the third cladding layer from laterally opposite sides thereof. This semiconductor layer device is of what we call a ridged structure.
In order to fabricate this ridged-structure semiconductor laser device, it is possible to use a fabricating method comprising:
a first process step of successively forming at least a first cladding layer, a quantum well active layer, a second cladding layer, and a third cladding layer on or above a semiconductor substrate, wherein a growth condition used for the second cladding layer is such that the second cladding layer has carbon serving as a p-type impurity at a concentration of 2xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923;
a second process step of processing the third cladding layer into a shape of ridge extending like a stripe; and
a third process step of forming an n-type current block layer on the ridge-shaped second cladding layer in such a manner as to sandwich the ridge-shaped third cladding layer in the current block layer.
By using this fabricating method, the impurity in the p-type second cladding layer can be prevented from diffusing into the quantum well active layer.
In one embodiment, in the first process step, a metal-organic chemical vapor deposition method is used and the growth condition for the p-type second cladding layer is that a molar ratio of a group V source to a group III source (V/III ratio) is 10 to 50. With this arrangement, the possible occurrence of the surface roughness of the grown crystal can be avoided, and at the same time, the second cladding layer achieves a carbon (C) concentration of 2xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923. Use of this fabricating method enables, with controllability and reproducibility superior to those in the conventional case, the p-type second cladding layer to have an impurity concentration that causes no inversion of the electric conduction type.
In the first process step for fabricating the ridged-structure semiconductor laser device, carbon may be employed as a p-type impurity for the third cladding layer, and a growth condition for the third cladding layer may be such that a carbon concentration of the third cladding layer is 2xc3x971017 cmxe2x88x923 or higher. In this case, the impurity in the p-type third cladding layer is prevented from diffusing into the p-type second cladding layer and the active layer. Thus, the diffusion of the impurity into the active layer is more effectively prevented. In this case, the molar ratio of the group V source to the group III source (V/III ratio) should preferably be set to 10 to 50 as a growth condition of the third cladding layer. With this arrangement, the occurrence of the surface roughness of the grown crystal can be avoided, and also, the third cladding layer can contain carbon (C) at the concentration of 2xc3x971017 cmxe2x88x923 or higher. This fabricating method enables, with controllability and reproducibility superior to those in the conventional case, the p-type third cladding layer to have an impurity concentration that causes no inversion of the electric conduction type.
The ridged-structure semiconductor laser device fabricating method may comprise, between the first and second process steps, a fourth process step of disordering the active layer in a light-emitting end surface portion of the semiconductor laser device through heat treatment to thereby form a window region.
As described above, according to the present invention, the p-type second cladding layer (and also the p-type third cladding layer in one embodiment) is doped with the impurity of carbon that diffuses very little. This arrangement can prevent the diffusion of the impurity from the p-type second cladding layer (also from the p-type third cladding layer in one embodiment) into the quantum well active layer, which will easily occur if the cladding layer is doped with an impurity of Zn instead, in this fourth process step. That is, the active layer is prevented from being disordered in the region (oscillation region) other than the window region.
The semiconductor laser device fabricated through this fourth process step has a window region in the light-emitting end surface portion, the window region having a band gap greater than that of the oscillation region. As described above, the disorder in the oscillation region of the active layer is suppressed, and therefore, the end surface window effect in this semiconductor laser device is great enough to allow the output power to be increased.
The fourth process step may include:
partially forming an SiO2 film or an SiN film on a wafer obtained through the first process step; and
thermally treating the wafer formed with the SiO2 film or the SiN film to disorder a portion of the active layer just below the SiO2 film or the SiN film.
When fabricating the semiconductor laser device having any one of the aforementioned structures, the p-type compound semiconductor layers can be formed of AlGaAs.
As is obvious from the above, the present invention can improve the yield of semiconductor laser devices and provides a semiconductor laser device having sufficient capabilities that is not inferior to the prior art products in terms of characteristics.
Other objects, features and advantages of the present invention will be obvious from the following description.